(First background art)
In manufacturing of a semiconductor device, in general, one of the important steps is micro-miniaturization processing. In this step, any desired characteristics of the semiconductor device can be obtained by electrically isolating elements from each other. For this purpose, conventionally, the device elements have been isolated electrically in accordance with a local substrate oxidizing method which is referred to as LOCOS (localized oxidation of silicon) method. In this LOCOS method, as shown in FIG. 4, a thick oxide film (field oxide film) 14 is formed locally on the basis of hydrogen combustion oxidation so that the elements can be isolated from each other electrically.
In more detail, as shown in FIG. 4(A), a pad oxide film 12 is formed on a Si substrate 11. Further, a nitride film 13 is formed at a part corresponding to a predetermined element forming region on the pad oxide film 12, as a mask against oxidation.
After that, as shown in FIG. 4(B), an oxide film 14 is formed locally on the basis of hydrogen combustion oxidation.
Successively, as shown in FIG. 4(C) the nitride film 13 and the thin oxide film (pad oxide film) 12 are etched at the middle portion thereof.
After that, as shown in FIG. 4(D), the whole substrate is oxidized again to form a field oxide film 14A and 14B between both the thick portions of the oxide film 14.
As understood by the above description, an element region can be formed being surrounded by the thick field oxide film 14. On the element regions thus formed, various MOS transistors and MOS capacitors are to be formed. With the advance of higher integration and microminiaturization of the devices, however, since the electric characteristics required for the devices have become severe more and more, various problems have arisen even in the LOCOS method. One of the problems is so-called bird's beak. Here, the bird's beak implies the bird's beak-like oxide film portion 12A extending so as to be opposed to each other on the element region, when seen in the cross section, as shown in FIG. (B). This bird's beak is formed, because oxygen infiltrates to under the nitride film 13 used as an oxidation mask when the field oxide film is selectively oxidized in accordance with the LOCOS method. Therefore, before forming the gate oxide film 14A, the bird's beak (the relatively thin oxide film 12A formed on the element region) must be etched off perfectly. In this pre-etching process, therefore, a sufficient etching processing time is determined in view of a margin in order to remove the oxide film 12A perfectly. In this case, however, since the field oxide film 14 is inevitably etched off slightly as shown in FIG. 4(C), corners 11A are produced at each end portion of the element region, with the result that the planarization of the element region deteriorates. Consequently, when the gate oxide film 14A is formed under these conditions, since the oxidization speed is slower at the corner portions than the oxidization speed at the other portions, the oxide film 14 is thinned locally (referred to as thinning), so that the thin portion 14B is formed. As a result, there exists a problem in that the oxide film is easily broken down starting from this thinned gate oxide film 14A.
(Second background art)
A number of methods of processing a semiconductor wafer by use of ultraviolet rays have been so far proposed. These methods are mainly of ozone oxidation method such that ozone or active oxygen is formed from oxygen in water or air by use of ultraviolet rays to remove organic substances from the surface of the semiconductor wafer by a strong oxidization performance of the formed ozone.
As the method of using ultraviolet rays other than ozone oxidation method, dry washing method has been so far known such as radicalization of various reactive gases. In the case of wet washing, on the other hand, only abstract expression has been so far made of the removal of inorganic substances, in addition to the above-mentioned ozone oxidation.
Further, the important parameters are metal contamination and particles adhering on the surface of the wafer from the standpoints of semiconductor wafer washing. In the ordinary process, since the semiconductor wafer is not largely contaminated from the outside, the removal of organic substances are limited to only the removal of resist. As a result, the wet washing has been often adopted. In the case of alkaline washing, since the surface of the semiconductor wafer can be etched, it is possible to remove particles effectively. In this method, however, there exist some problems in that it is difficult to prevent metal from being absorbed by the wafer surface or the wafer surface roughness from being deteriorated. In contrast with this, in the case of acid washing, although metal contamination can be removed effectively, since the surface of the semiconductor wafer cannot be etched, particles cannot be removed effectively. As a result, conventionally, the alkaline and acid washing have been often used in combination. However, this washing method is not an optimum washing method.
On the other hand, as the metal impurity analysis techniques of the semiconductor wafer, various high sensitivity methods have been so far developed in both surface analysis and bulk analysis. In the case of the surface analysis, for instance, TRXRF (total reflection X-ray fluorescent spectroscopy) or VPD (vapor phase decomposition method) and/or AAS (atomic absorption spectroscopy) (e.g., vapor phase cracking atomic absorption spectroscopy) are in particular effective as the high sensitivity analysis. Further, in the case of the bulk analysis, there have been adopted such methods that: the surface layer of a semiconductor substrate is etched by use of a mixture liquid (hydrofluoric and nitric acid) of hydrofluoric acid and nitric acid, and further the heated and concentrated etching liquid is analyzed in accordance with the AAS method or such that: a piece of the semiconductor wafer is dissolved by a full hydrofluoric and nitric acid solution and the dissolved wafer is analyzed in accordance with AAS (flame-less atomic absorption spectroscopy) of ICP (induction coupled plasma) and/or MS (mass spectroscopy) (i.e., induction coupled plasma mass spectroscopy).
In practice, however, the distribution of metallic impurities in the semiconductor wafer largely differs according to the thermal process immediately before the analysis. In addition, in many cases, there exists such a tendency that the metallic impurities exist the most in the outermost surface thereof and decrease gradually in the direction toward bulk center. According to circumstances, metallic impurities are trapped and deposited at the crystal defects localized near the bulk center, so that the distribution of metallic impurities differs largely in the depth direction. In other words, in order to know the distribution of the metallic impurities of the semiconductor wafer, it is important to analyze the etchent used for etching the semiconductor surface. On the other hand, in general, since the semiconductor wafer is etched by use of hydrofluoric nitric acid based etchent, a relatively high concentration acid is required to analyze the surface of the semiconductor wafer. Further, since the reactivity of the etchent is high, the reaction starts immediately after the contact of the etchent with the semiconductor wafer. As a result, the etching is not effected smoothly, and in addition a high purity chemicals is essentially required. In particular, where the outermost layer of about 10 nm is required to be analyzed, the analyzed value is largely subjected to the influences of the impurity of the chemical and the uniformity of the etching.
(Third background art)
The monocrystal wafer used for the MOS type semiconductor substrate is formed by first melting polycrystal silicon within a quartz crucible, by manufacturing an monocrystal ingot in accordance with a pulling method referred to as CZ (Czochralski) method, and further by cutting the ingot into a wafer. The polycrystal silicon used as raw material is formed by reducing quartzite into solid silicon and by further refining the solid silicon through several stages. The formed polycrystal silicon is melted in a quartz crucible as raw material for pulling a monocrystal silicon. Before being placed in the quartz crucible, the surface of the polycrystal silicon is etched about 1 to 5 .mu.m by a mixture liquid of acid and nitric acid in order to remove impurities from the surface of the polycrystal silicon.
The purity of the polycrystal silicon used for the prior art technique is very high. In general, however, the silicon wafer formed by use of the high-purity polycrystal silicon includes metallic impurities enough to deteriorates the device characteristics. As a result of analysis of the liquid obtained by dissolving the silicon wafer by a mixture liquid of acid and nitric acid in accordance with the flame-less atomic spectroscopy, it was known that there exist Fe of 10.sup.10 atoms/cm.sup.3 and Al of 10.sup.11 atoms/cm.sup.3. As already explained, although the impurities on the surface of the polycrystal silicon can be removed, the metallic impurities contained in the polycrystal silicon can not be removed in practice.
As described above, in the prior art method, since there exists a thin portion in the gate oxide film, there exists a problem in that the gate oxide film is easily broken down beginning from this thin portion thereof. In addition, there exist other problems in that it is difficult to wash the semiconductor wafer easily and further analyze the metallic impurities of the semiconductor wafer. Further, although the impurities on the surface of the polycrystal silicon can be removed, there exists a problem in that it is difficult to remove the metallic impurities in the polycrystal silicon.